Background: Numerous studies have demonstrated lung cancer risk from high-level radon exposure in underground miners. Data on moderate or low levels of exposure are sparse. Public interest in the possibility of lung cancer risk from residential exposure to radon in US homes has prompted the development of risk estimates for residential exposure that are based on extrapolating from models derived from pooled data from studies of miners. Few studies of miners include actual data on effects at exposures at levels that might be found in homes. Studies of miners have been inconclusive with regard to cancers other than the lung, although there is suggestive evidence for increased risk of hematopoietic and digestive cancers. Prior studies have largely been restricted to analysis of mortality and may therefore underestimate the risk for cancers with relatively high survival rates. Summary: We investigated cancer incidence in a cohort of 16,434 underground uranium miners from the Czech Republic. Eligible miners worked at least one year underground prior to January 1, 1976, and were alive and living in the Czech Republic on January 1, 1977. The cohort was followed for cancer incidence and mortality through December 31, 1992 by linking with the Czech cancer registry. Detailed records on daily work location and radon levels, smoking, dusts, and gamma radiation were obtained for a 2,000-person stratified random sample of the cohort and all cancer cases for use in case-cohort analyses. Cancer follow-up was extended through 1998 to increase the number of miners with cancers of sites other than the lung for inclusion in case-cohort analyses. Lung cancer was diagnosed in 752 miners through 1992. Fifty-six percent of miners began mining after 1960 when industrial hygiene measures substantially reduced radon exposure. Cumulative exposure was less than 50 WLM for 72% of the cohort. Miners had a more than two-fold risk of developing lung cancer. Age- and calendar year-adjusted relative risks comparing exposed miners to those with fewer than 5 WLM showed increasing risk with increasing levels of exposure. Preliminary case-cohort analysis confirms that the radon-associated risk for lung cancer is not explained by smoking or dust in the mines. Cancer of sites other than the lung was diagnosed in 1100 miners. Risk was increased for some cancers including leukemia, and cancers of the larynx, pancreas, liver, stomach, and colon and rectum, but the absence of consistent dose-response trends for cancers other than hematopoietic cancers points to factors other than radiation. In a preliminary case-cohort analysis of risk for hematopoietic cancers, radon and gamma irradiation appear to be independent risk factors for hematopoietic cancers in general and leukemia specifically. Although chronic lymphocytic leukemia (CLL) has been thought to be the only leukemia subtype that is not related to radiation, risk for CLL was significantly increased at exposures above 50 WLM for radon and 20 mSv for gamma radiation versus below and in a continuous analysis. Our data provide much needed information on exposure levels that are more reflective of exposures found in residences. We are the first to study cancer incidence among underground miners rather than mortality and will have the opportunity to directly compare the results when incidence or mortality data are used.